The escalating costs of energy have intensified efforts to develop alternate energy sources. One result of this effort is the integrated gasifier combined cycle power plant.
The generation of electricity by integrated gasifier combined cycle power systems offers the possibility of reduced power cost and lower environmental impact than standard coal-fired power plants. In these advanced systems, coal or other carbonaceous material undergoes a partial oxidation gasification reaction with oxygen that usually has a purity of at least 80 volume % oxygen. The gas produced is cleaned to yield a low-sulfur synthetic fuel gas. This synthetic fuel gas which comprises mainly hydrogen and carbon monoxide can be utilized in a gas turbine generation system to produce electric power with reduced environmental emissions.
The increased interest in gasification combined cycle technology in recent years has been stimulated by the higher efficiency and demonstrated reliability of advanced gas turbines, coal gasification processes, and air separation systems which are utilized in the integrated gasifier combined cycle systems. The proper integration of these three main components of an integrated gasifier combined cycle system is essential to achieve maximum operating efficiency and minimum power cost.
The integrated gasifier combined cycle system is described in more detail in U.S. Pat. No. 4,328,008 to Munger et al., and U.S. Pat. No. 4,052,176 to Child et al. The disclosure of these patents is incorporated herein by reference.
Combustion-based power generation systems, including integrated gasifier combined cycle systems, are subject to periods of operation below system design capacity due to changes in demand for electric power. During these periods, such systems operate below design efficiency. The equipment selection and process design of an integrated gasifier combined cycle system therefore must address steady-state operation at design capacity as well as operation at off-design, part load, or turndown conditions.
An air- and nitrogen-integrated gasifier combined cycle system is a preferred option because of the potential for operating such a system at maximum overall efficiency, particularly when the system also must operate at off-design, part load, or turndown conditions.
Because the operation of such a plant depends on consumer demand for electricity, the oxygen input to the plant often needs to vary along with the electricity demand and the reduction in power demand which occurs in the typical daily power demand cycle. For example, the nighttime power demand on a typical integrated gasifier combined cycle plant can be 50-75% of the daytime demand. Seasonal changes in power demand also may occur. During reduced power demand, the plant must be operated at part load, i.e. "turned down" by decreasing the flow of air and fuel to the gas turbine combustor.
The output variation of the integrated gasifier combined cycle system corresponds to either an increased or decreased need for products from the air separation unit which produces oxygen and nitrogen for use in the system, most importantly, the quantities of oxygen needed for the gasifier operation. Also, it is important that during increased or decreased production by the air separation unit, the purity of the products remain at or above the levels required by the gasification process.
Unfortunately, a problem is created by integrating the air separation unit with the integrated gasifier combined cycle system. Prior to the advent of the integrated gasifier combined cycle system, air separation units did not have to vary their production as severely as the operation of an integrated gasifier combined cycle requires, and they were designed accordingly. Demands typically placed on a fully integrated air separation unit are such that it must be capable of operating in the range of 50% to 100% of design capacity while responding to variations in production rate, sometimes referred to as "ramping", at about 3% of capacity per minute.
To illustrate the problem, during partial load operation or "turndown" of the air separation unit, less product is needed, yet liquids in the distillation columns flash as the air supply pressure decreases tending to generate more product. Also, the flashing liquid is oxygen rich which can potentially degrade the purity of the oxygen and nitrogen product streams.
The problem then arises as to how to control the variations in an air separation unit which may have a varying compressed feed air pressure, while meeting varying demands for oxygen and strict purity requirements.
It would be desirable for the air separation unit to have the oxygen production capacity to meet peak load requirements while not operating at sub-optimum level during off peak periods, since the efficiency of the air separation unit decreases when not operating at or near its design capacity. It would also be desirable to be able to increase power generation to super-design levels during peak periods without incurring additional costs from oversized equipment or non-optimum operating conditions.
The objective is to find a technique to permit the air separation unit to produce oxygen at an efficient level despite fluctuations in requirements resulting from the variation of demand for electricity for an integrated air separation unit, while maintaining a reasonably constant purity to satisfy the criteria of the gasifier of the integrated gasifier combined cycle power generation system.
U.S. Pat. No. 5,526,647 to Grenier, incorporated herein by reference, discloses a process for producing gaseous oxygen under pressure at a variable flow rate utilizing an liquid air storage vessel and an liquid oxygen storage vessel.
Incoming air is cooled in a heat exchanger by heat exchange with products from a distillation apparatus. Liquid oxygen is withdrawn from the distillation apparatus, brought to vaporization pressure, vaporized and reheated in the heat exchanger by incoming air which is thereby liquefied.
During a reduction in demand for gaseous oxygen under pressure relative to the nominal flow rate, excess liquid oxygen produced by the distillation apparatus is withdrawn and sent to a liquid oxygen storage vessel. A quantity of liquid air previously stored, corresponding in amount to the liquid oxygen withdrawn, is introduced into the distillation apparatus.
During an increase in demand for gaseous oxygen under pressure relative to the nominal flow rate, the required excess oxygen is withdrawn in liquid form from the liquid oxygen storage vessel, brought to vaporization pressure, and vaporized under this pressure in the heat exchanger. A corresponding quantity of air is stored which has been liquefied by such vaporization, in the liquid air storage vessel.
Disadvantages of this system include the necessity of providing two storage vessels, one for liquid air, the other for liquid oxygen, and providing lines and pumping means for transporting such liquefied gases.
Storing oxygen in the form of a gas or liquid in external tanks entails high capital costs. Storing liquid O.sub.2 outside the refrigeration section or cold box of the air separation unit imposes large refrigeration costs to maintain the temperature at the proper level.
U.S. Pat. No. 5,265,429 to Dray, incorporated herein by reference, accommodates the varying load on the plant by using a product boiler to generate gaseous O.sub.2 from liquid O.sub.2 coupled with the use of a liquid air storage tank between the product boiler and the cryogenic rectification to solve both the loss of refrigeration caused by liquid oxygen withdrawal from and operating fluctuations in the cryogenic rectification plant.
U.S. Pat. No. 5,437,160 to Darredeau, incorporated herein by reference, relates to air separation units where the oxygen produced is used in a integrated gasifier combined cycle power system.
Darredeau proposes solving the problem of varying demand for oxygen by introducing an excess of liquid rich nitrogen into the distillation apparatus when the demand for product or the flow rate of the supplied air increases and by withdrawing an excess of liquid rich nitrogen from the distillation apparatus and storing this liquid when the demand for product or the flow rate of the supplied air decreases.
The present invention addresses the need for improved methods to operate advanced power generation systems, and in particular describes the improved operation of air- and oxygen-integrated integrated gasifier combined cycle system and air separation unit systems at various load conditions.